1. Field of the Invention
The present invention relates generally to a method and apparatus for estimating a carrier-to-noise ratio in a wireless access system. More particularly, the present invention relates to a method and apparatus for estimating a carrier-to-noise ratio using a preamble signal in an environment where the service coverage of a Base Station (BS) is extended.
2. Description of the Related Art
The Institute of Electrical and Electronics Engineers (IEEE) 802.16 standard defines a wireless access system that enables high-speed data transmission with mobility. According to a Self Organized Network (SON) technology, wireless environment information is acquired by estimating the Carrier-to-Noise Ratios/Carrier-to-Interference and Noise Ratios (CNRs/CINRs) of neighbor BSs and optimum operation parameters, such as transmission power control, Frequency Allocation (FA) selection, etc., are updated autonomously based on the CNRs/CINRs of the neighbor BSs.
A BS estimates a timing offset using a preamble and then estimates a CNR/CINR using the timing offset.
The structure of a preamble signal for CNR/CINR estimation will be described. The first Orthogonal Frequency Division Multiplexing (OFDM) symbol of a frame is allocated to a preamble. A preamble sequence is allocated to every third subcarrier in the OFDM symbol.
FIGS. 1A, 1B and 1C illustrate methods for allocating a preamble signal in a wireless access system according to the related art.
Three methods for allocating a preamble sequence may be defined according to the positions of subcarriers allocated to the preamble sequence. FIGS. 1A, 1B and 1C illustrate these three methods which are referred to as segment 0, segment 1, and segment 2, respectively. That is, a segment refers to a method for allocating subcarriers to a preamble signal. A preamble sequence is repeated at every third tone in each segment. For example, a preamble sequence is repeated at tones 0, 3, 6, . . . in segment 0 (FIG. 1A), at tones 1, 4, 7, . . . in segment 1 (FIG. 1B), and at tones 2, 5, 8, . . . in segment 2 (FIG. 1C). In segment 0, tone 426 is not allocated.
FIG. 2 illustrates a method for estimating a timing offset by Inverse Fast Fourier Transform (IFFT)-processing the frequency response of a preamble sequence according to the related art.
When a preamble sequence having a Preamble Index (PI) is IFFT-processed, the components of the preamble sequence are delayed by a specific number of samples according to the arrival time of the preamble sequence in the time domain. In FIG. 2, a Fast Fourier Transform (FFT) size is 1024, by way of example. Because the preamble sequence is repeated at every third tone, the components of the IFFT signal is repeated every 341 samples.
In general, a pico cell BS estimates a timing offset only for a signal having fewer than 341 samples except for a signal repetition period, using a search window 200 of a size smaller than the repetition period (i.e., 341). The estimated timing offset indicates the start position of FFT samples and is used to estimate a carrier power and the Received Signal Strength Indication (RSSI) power and noise power of each segment through FFT. Especially, the estimated timing offset is used to estimate the CNR/CINR of the preamble index.
                              CNR          PI                =                              RxR            PI                    NP                                    (        1        )                                          CINR          PI                =                              RxR            PI                                              I              PIseg                        +            NP                                              (        2        )            
where R×RPI denotes the carrier power of the preamble, IPIseg denotes interference except for the segment RSSI power of the preamble index in the RSSI power of a segment having the preamble index, and NP denotes a noise power.
A BS using the SON-based self-configuration technology should measure signals from neighbor BSs to acquire information about the wireless channel environment of the neighbor BSs in the IEEE 802.16 system. Thus, the BS can optimize operation parameters based on the acquired information.
The signals from the neighbor BSs are delayed according to the distances to the neighbor BSs or the adjacent wireless channel environment. Therefore, the BS corrects a timing offset to synchronize with the delayed signals by estimating the timing offset using a preamble signal.
As illustrated in FIG. 2, in view of the characteristic of a preamble sequence being repeated every three tones, the IFFT signal of the preamble sequence is repeated every 341 samples in the time domain. Therefore, if the search window size is increased to a sample period being a third of the FFT size (i.e., 341 or more samples) as indicated by reference numeral 202, an actual preamble signal is not distinguished from its repetition when a peak component of the preamble signal is located. As a result, the use of a search window having as many samples as a third of an FFT size leads to the degradation of estimation performance.
However, because the IEEE 802.16 system using the SON technology needs to increase a search window size to or above a third of an FFT size due to various requirements of service coverage and transmission power, there is a need to provide an improved method and apparatus for estimating information about an adjacent wireless environment using a CNR in a wireless access system.